Actually, according to the published sources (i.e. the article whose Zelphic sectionI summarised) the Primacy could not have been founded in Imperial 5, as it states:
"By the beginning of the Third Imperium there were a dozen planets colonised by humans in the Far Frontiers sector. This number rapidly increased over the next few centuries until 536 Imperial, when the first human state in the sector was formed, the Protectorate."